Determination of Fluorine in Wine Modification of the Willard and Winter Method H. G. REMPEL The Twining Laboratories, Fresno, Calif. The distillate was neutralized with a saturated solution of sodium carbonate, so as to make it alkaline to phenolphthalein, and was then evaporated down to a volume of about 10 ml. During the evaporation the temperature of the distillate should not exceed 85" After cooline. the samtde was neutralized with a I to 20 hvdrochloric acid soTution, so 'as just to discharge the color of pgenolphthalein. To the colorless solution, 1 ml. of 0.05 per cent aqueous solution of sodium alizarin sulfonate was added, which serves as the indicator in the subsequent titration. This indicator was roposed by Armstrong ( 2 ) to replace the zirconium nitrate anzalizarin red mixture used in the original Willard and Winter (12) method. The resulting red color was changed to a golden yellow by adding a 0.01 N hydrochloric acid solution drop by drop. An excess of only a few drops will interfere with the titration. After this adjustment the solution should have an acidity of about 5.0 pH. At this point the solution was made up to a volume of 100 ml. and divided into two equal parts, in order to get a double check on the titration. To each aliquot an equal volume (50 ml.) of 95 per cent alcohol was added. The water used for the dilution as well as the alcohol should have an acidity of about 5.0 pH. Denatured alcohol was found to serve the purpose. The solution was then titrated with a 0.01 N solution of thorium nitrate. A small portion of the thorium nitrate solution is used up by the sodium alizarin sulfonate indicator. The correction amounts to 0.1 ml. of 0.01 N thorium nitrate for the 0.5 ml. of indicator used in each of the two aliquots. The titration was conducted by first adding 0.1 ml. of the thorium nitrate solution-the amount required by the indicator. A sharp end point at this time indicated the absence of fluorine. If no end point was reached, the fluorine was titrated by adding the thorium nitrate slowly drop by drop until the end point developed. The sample should be stirred vigorously and be permitted to stand a t least 15 seconds after each addition. The volume of thorium nitrate used above 0.1 ml. corresponded to the fluorine in the sample, and the value for each milliliter was obtained from the standardization against a known sodium fluoride solution. Each aliquot represents 25 ml. of wine. Results may be reported as milligrams of fluorine per liter of wine. By taking the specific gravity of the wine, the results may be calculated as parts per million or grains per pound.
D
U R I S G the last few years the analytical chemist has often been called upon to determine the fluorine content of various food products. It is often necessary to establish whether the fluorine content is below the tolerance of 0.01 grain per pound as required by the United States Food and Drug Administration. Since fluorides and fluosilicates are being used more extensively as insecticides and dusting powders on various crops, the possibility of fluorine contamination is steadily increasing. The analytical chemist in a commercial laboratory is expected to produce reliable results in the shortest possible time and must have practical methods at his disposal. Numerous contributions on the subject of fluorine determination have appeared in the scientific literature, particularly since the publication of the Willard and Winter method in 1933 ( I d ) . Many precautions have been pointed out and a number of valuable improvements have been suggested. If all these suggestions were followed a very lengthy and tedious method would result. I n the course of examining nearly a thousand samples of wine for their fluorine content, the author's laboratories found the following procedure to give the most satisfactory results. The method has been simplified as much as possible, without impairing the accuracy of the results.
e.
Reagents Saturated sodium carbonate solution Perchloric acid, 60 per cent Hydrochloric acid (1 to 20, 5 per cent by volume) Hydrochloric acid, 0.1 N Aqueous solution of sodium alizarin sulfonate, 0.05 per cent Ethyl alcohol (or denatured alcohol), 95 per cent Thorium nitrate solution, 0.01 N Standard sodium fluoride solution
Procedure
Ashing Temperature Since wine contains a considerable amount of sugar, i t was necessary to char the sample by applying an open flame t o
Fifty milliliters of wine were measured into a platinum dish, neutralized with a sodium carbonate solution, and evaporated to near dryness on a water bath. The sample was completely freed from moisture by the application of an open flame to the surface of the solids in the dish. (The flame must be kept, in continuous motion so as to avoid overheating any portion of the dish, and the heating discontinued as soon as the foaming ceases.) The sample was then burned in a thermostatically controlled electric muffle at a temperature of 525" C. The ash from the wine was washed from the platinum dish into a 125-ml. distilling flask. Ten milliliters of water had been previously added to the ash for the purpose of dissolving and loosening it from the sides of the dish. The water used for transferring the ash should not exceed 25 ml. The flask was then connected with a condenser and equipped with a thermometer reaching within 0.5 cm. of the bottom of the flask. Twelve milliliters of 60 per cent perchloric acid were added and the contents steam-distilled by the Willard and Winter (12) method at a temperature between 130' and 140" C. The distillate was collected in a 400-ml. beaker. During the distillation the flask was immersed in an oil bath maintained a t about 150" C., as suggested by Reynolds (9). The flow of steam was regulated so as to keep the tem erature in the distilling flask as nearly as possible a t 135" C. an: to collect 200 to 250 ml. in 4.5 to 60 minutes. If the tem erature drops below 130' C. for a short period during the distilition, a larger distillate should be collected. If the rate of distillation is slow, the time should be extended so as to collect a minimum of 200 ml. The flow of steam was regulated by increasing or decreasing the heat under the flask providing the water for the steam distillation.
the surface of the solids left after drying on the water bath. If this is not done excessive foaming will result when the sample is introduced into the hot furnace. Samples were burned in thermostatically controlled muffle furnaces at 525" C. This was the lowest temperature at which the resulting carbon could be satisfactorily ashed. The time required for ashing usually is 6 to 12 hours, although some samples require as long as 16 hours. Experimentation has shown that the temperature of ashing should be kept as low as possible. Winter and Butler (14) ashed their samples a t dull red tem'perature, and Hoskins and Ferris (6) reported a considerable loss of fluorine when the sample was heated u p to 800" C. even for a short period of time. One of the experiments conducted in the author's laboratories relative to ashing temperatures consisted in subjecting a solution of sodium carbonate, t o which a known amount of sodium fluoride had been added, to temperatures ranging from 450" t o 800" C. Results are shown in Table I. A considerable loss of fluorine was observed even at 500" C., only a trace of fluorine being left in the sample which was heated u p to 800" C. This indicates the necessity of a suit378
JULY 15, 1939
ANALYTICAL EDITION
TABLEI. EFFECTOF ASHINGTEMPERATURE ON FLUORINE RECOVERY (Sodium carbonate solution with sodium fluoride added) Ashing Fluorine Fluorine Temperature Added Recovered Loss
c. 450 500 550 600 650 700 750 800
.
Mg.
Mg
0.452 0.452 0.452 0.452 0.452 0.452 0.452 0.452
0.452 0.386 0.262 0.196 0.129 0.001 0.063 Trace
% None 14.6 42.0 56.6 71.4 80.0 86.1 95
+
TABLE11. FIXATIVE EFFECTOF WINEASH (50 ml. of wine with sodium carbonate and sodium fluoride added) Ashing Fluorine Fluorine Recovered Loss Temperature Added
c.
450 500 550 600 650 700 750 800
Mg.
Mg.
%
0.452 0.452 0.452 0.452 0.452 0.452 0.452 0.452
0.452 0.461 0.443 0.452 0.452 0.424 0.386 0.196
None None 2.0 None None 6.2 14.6 56.6
able fixative in the ashing of organic matter when fluorine is to be determined.
379
used by the majority of investigators. Shuey (11)and others used 95 per cent sulfuric acid, particularly when organic matter was present, because of the explosive properties of perchloric acid in the presence of organic matter. Reynolds (8) suggests the use of phosphoric acid. The author found 60 per cent perchloric acid to give satisfactory results in the distillation of fluorine from wine ash. A 125-ml. distilling flask was substituted for the conventional 50-ml. Claissen flask. No porous platen or glass beads were introduced into the flask, since wine ash contains a considerable amount of silica. The wine must be thoroughly ashed. Incompletely burned samples cause frothing and consequent carrying over of nonvolatile portions of the ash. The distillate collected by Willard and Winter (12) was 50 to 75 ml. Reynolds (9) increased the volume to 150 ml.; Hoskins and Ferris (6) reduced the loss of fluorine to a negligible point by collecting 200 ml.; the author collected 200 to
TABLE111. ANALYSISOF WINEASH %
% Silicon oxide (SiOl) Aluminum oxide (A1203) Iron oxide (FesOa) Calcium oxide (CaO) Magnesium oxide (MgO) Potassium,oxide (KzO) Sodium oxide (NazOj Sulfuric anhydride (so3)
0.82 0.25 0.32 7.81 6.36 49.22 1.65 7.27
Phosphoric acid (PzOsj 10.35 Chlorine (C1) 1.15 Carbon dioxide (COz) 14.66 Manganese (Mn) Trace Cop er (Cu) Trace Lea1 (Pb) Trace Titanium (Ti) Faint trace
Fixative Effect of Wine Ash
It was found that the ash of wine serves as a fixative for fluorine. While considerable loss of fluorine occurred from dried alkaline sodium fluoride solutions, no loss was observed when the fluorine was added to wine. Table I1 shows that no loss of fluorine occurred up to 650" C. An analysis of the ash from a mixture of four different kinds of wine was made to determine the constituents responsible for the fixative properties of wine ash (Table 111). This ash represents 0.25 per cent of the wine by weight. It was found to contain about 8 per cent of calcium oxide and about 6 per cent of magnesium oxide. Although aluminum salts possess fixative properties they appeared in only very small amounts. Dahle (4) used lime in the process of ashing plant material for fluorine determination. Winter (13) suggested the use of magnesium acetate, which was also used by McClure ( 7 ) . The amounts of fixative used by these authorities were generally high, usually above 0.5 gram. I n making a number of experiments in regard to the fixative values of magnesium oxide and calcium oxide, both of which were found in the wine ash, the author was able to establish that magnesium oxide when used in small amounts has very little fixative effect o n fluorine. At least 0.25 gram of magnesium oxide was required to fix 0.45 mg. of fluorine a t 600" C., while as little as 0.05 gram of calcium oxide was sufficient to prevent the loss of 0.45 mg. of fluorine a t 500" C. as well as 600" C. (Table IV). Slightly high results in the fluorine recovery shown in Table IV were due to the fact that the calcium oxide used in this experiment contained small quantities of fluorine. The author's investigations show that as long as only small amounts of fluorine are present, the ash of the wine prevents its loss by ashing a t 525" C. This makes it possible t o simplify the procedure by leaving out the fixative, which can easily be the source of fluorine contamination and lead to an error. Distillation I n the original Willard and Winter (16) method 60 per cent perchloric acid was used for the volatilization of fluorine as hydrofluosilicic acid. Since that time perchloric acid has been
TABLEIV. FIXATIVE EFFECTO F CALCIUM OXIDE AT 500' AND 600" C.
0.452 0,452 0.452 0.452
CaO Fluorine Added Recovered Oram Mg. Heated for 4 Hours at 500' C. None 0,386 0.05 0.461 0.1 0.461 0.25 0.471
0.452 0.452 0,452 0.452
Heated for 4 Hours at 600' C. None 0.196 0.05 0.461 0.1 0.452 0.25 0.461
Fluorine Used
Mg.
Op; FLUORINE
Loss
% 14.6 None None None 56.6 None None None
250 ml. The time required for each distillation was from 45 to 60 minutes and the optimum temperature 135" C. If the temperature during the distillation dropped temporarily below 130" C., a larger distillate was collected. A number of ions, particularly phosphates, sulfates, and chlorides, interfere with the subsequent titration. Reynolds (8),Gilkey (6), and Churchill (3) report interference from the phosphate ion. Churchill (3) was able to overcome the difficulty by a double distillation using first sulfuric and then perchloric acid. Hoskins and Ferris (8) and Armstrong (1) found chlorides t o interfere with the titration. This interference was eliminated by McClure (7) and others by adding a silver salt to the distilling flask to fix the chlorides as silver chloride. Distillates obtained from wine ash as described above, by a single distillation over perchloric acid, were shown to be free from sulfates and phosphates, by making specific tests for those ions on a number of distillates. The use of a 125-ml. distilling flask possibly accounts for this fact. The distillates were found to contain a faint trace of chlorides, but not sufficient to interfere with the titration of fluorine with thorium nitrate. The distillate, which had been collected in a 400-ml. beaker, was neutralized with a saturated solution of sodium carbonate, so as to make it definitely alkaline to phenolphthalein, and mas then evaporated on a hot plate down to about 10 ml. The evaporation should take from 4 to 6 hours, the tempera-
INDUSTRIAL -4ND ENGINEERING CHEMISTRY
380
ture not exceeding 85" C. S o loss of fluorine due to evaporation in glass containers mas observed, as reported by McClure (7).
Titration The most delicate step in the determination of fluorine is the adjustment of the acidity previous to the titration. It affects the sharpness of the end point as well as the amount of thorium nitrate required to compensate for the sodium alizarin sulfonate indicator used for the titration. Rowley and Churchill (10) in making fluorine determinations on aqueous solutions with 0.1 -1;thorium nitrate acidified the solution to a p H of 2.9 to 3.1 before titrating. Hoskins and Ferris (6) used a buffer consisting of sodium hydroxide and chloroacetic acid to control the acidity during the titration a t 3.5 pH. After considerable experimentation in regard to the sharpness of the end point, the amount of indicator to be used, and the optimum acidity for the titration, the following procedure was found to be very satisfactory: The concentrated distillate was first neutralized with a 1 to 20 hydrochloric acid solution, so as to discharge the color of phenolphthalein. After the addition of 1 ml. of indicator the acidity was carefully adjusted by adding a weak (about 0.01 N ) hydrochloric acid solution drop by drop until the red color of the indicator changed to a golden yellow. Since the titration with thorium nitrate is extremely sensitive to the acidity of the solution, the addition of the acid should be discontinued just as soon as the golden yellow color is reached. The solution was then made up to a volume of 100 ml. and divided into two equal parts; 50 ml. of alcohol were added to each of the two aliquots. Titrations made on solutions thus prepared gave a sharp end point and yielded results which were in agreement with added amounts of fluorine. pH determination made on a large number of solutions prepared as described gave an acidity ranging from pH 4.6 to 5.3 with an average of pH 5.0. The alcohol added before the titration should have the same acidity. Alcohol with excessive acidity should be adjusted to pH 5.0 with sodium carbonate, and perfectly neutral alcohol should be slightly acidified with hydrochloric acid. Another important factor for a successful titration is the amount of indicator used. Since a portion of the standard thorium nitrate solution is used up by the sodium alizarin sulfonate, the amount of the indicator should be kept constant. I n the author's procedure 1 ml. of indicator was used, or 0.5 ml. in each of the two aliquots. This amount required slightly less than 0.1 ml. of 0.01 N thorium nitrate, yielding a sharp end point if no fluorine was present. The volume of the standard thorium nitrate used for the titration, less the 0.1 ml. necessary to compensate for the indicator, was in close agreement with added amounts of fluorine. The titration was conducted on white porcelain or paper. The color of the end point should be the same as is produced by 0.1 ml. of 0.01 N thorium nitrate on a solution containing no fluorine, when 0.5 ml. of indicator is used.
No attempt was made to read the volume of thorium nitrate used in the titration closer than 0.05 ml., one drop being added at a time. This naturally makes the fluorine corresponding to 0.05 ml. of standard thorium nitrate solution the smallest amount to be determined by this procedure. One milliliter of a perfectly 0.01 N thorium nitrate solution equals 0.19 mg. of. fluorine, which makes 0.0095 mg. or 9.5 micrograms of fluorine the limit of accuracy and the smallest amount to be determined. Summary A simplified procedure for the determination of fluorine in wine has been worked out by applying the principles of the Willard and Winter (12) method. It was found that a fixative need not be added previous to the ashing of the sample, since wine ash serves as a fixative for fluorine, thus eliminating a possible chance of error. By the use of a 125-ml. distilling
VOL. 11, NO. 7
flask a single steam distillation over perchloric acid yielded a distillate free from interfering ions. A sharp end point was obtained by carefully adjusting the acidity of the solution prior to the titration with thorium nitrate giving consistent and reliable results. While this is not a micromethod, it can be used to determine as little as 0.01 mg. of fluorine.
Literature Cited Armstrong, W. D., IKD. ENG.CHEM.,Anal. Ed., 8, 384 (1936). Armstrong, W. D., J . Am. Chem. SOC.,55, 1741 (1933). Churchill, H. V., Bridges, R. W., and Rowley, R. J., IND.ENQ. CHEM.,Anal. Ed., 9, 222 (1937). (4) Dahle. Dan. J . Assoc. Oficial Aor. Chem.. 18. 194 11935). i 5 j Gilkey, W. 'K., Rohs, g.L., and Hansen, H.V.; IND. ENG. CKEM.,Anal. Ed., 8, 150 (1936). (6) Hoskins, W. M., and Ferris, C. A., Ibid., 8, 6 (1936). (7) McClure. F. J., Ibid.. 11, 171 (1939). (8) Reynolds, D. S., J . Assoc. Oficial AQT.Chem., 18, 108 (1935). (9) Ibid.. 19. 156 (1936). (10j Rowley, 'R, J.,'and'Churchill, H. V., IND.ENG. CHEM.,Anal. Ed., 9, 551 (1937). (11) Shuey, G. A., J . Assoc. Oficial Agr. Chem., 18, 156 (1935). (12) Willard, H . H., and Winter, 0. B., ISD.ENG.CHEM.,Anal. Ed., 5. 7 (1933). (13) Winter,'O. B., J . Assoc. Oj'icial Agr. Chem., 19, 359 (1936). (14) Winter, 0. B., and Butler, L., Ibid., 16, 105 (1933).
An Efficient Defoaming Agent PHILEAS A. RACICOT
AND
CARL S. FERGUSON
Massachusetts Department of Public Health, Boston, Mass.
I
N CERTAIN food and biological chemical analyses foaming causes difficulties, especially when viscous substances in water are used at room temperatures. In such analytical procedures as the aeration of eggs and meats for the determination of free ammonia in estimating the degree of protein decomposition, paraffin oil is a good defoaming agent, but i t does not prevent foaming in the aeration of oysters, clams, and scallops. Capryl alcohol was tried and, although better than paraffin oil, i t had the disadvantage of causing a precipitate with Nessler's reagent. Phenyl ether was not as good as capryl alcohol in the prevention of foam and had the same disadvantage in nesslerization. Octyl alcohol (2-ethylhexanol) was tried and found to be a good defoaming agent, having no effect on Nessler's reagent, but was unsatisfactory for a 2-hour run because i t volatilized too rapidly, being effective for only from 10 to 15 minuttes. Experiments with octyl alcohol, mixed with various substances which would reduce its vapor tension and possibly increase its defoaming property, resulted successfully with the following mixture: Hard p a r a 5 n Heavy p a r a 5 n oil (U. 5. P. liquid petrolatum) Octyl alcohol
1 part 1 part
2 parts (by volume)
Heat the paraffin and paraffin oil until the paraffin is melted. At this point, stop heating and add the octyl alcohol, mix well, and pour into glass jars or wide-mouthed bottles and allow to cool. For use in aerometric determinations of free ammonia, 1 gram (this is not critical) in each cylinder works very well. The cylinders used in the authors' work have an inside diameter of 3.5 cm., are 28.5 cm. tall, and should contain about 60-cc. total volume of the mixture being aerated. PRESENTED before the Division of Agrioultural and Food Chemistry at the 97th Meeting of the American Chemical Society, Baltimore, Md.
